When there is a drought, how do you know if water in a pond or lake is too concentrated to be safe to swim in?

I think you are wise: to question this. The most common illnesses contracted by lake swimmers are infections of the skin, eyes, ears, nose and throat. Unfortunately, there is no simple and reliable way of determining if waters are free of contaminants without laboratory testing. Posting that water is "safe" or "unsafe" is, obviously, very helpful. Beyond this, I don't think there is any reliable way to know. Good luck

You don't...: Obviously, if the water has an odor or looks dirty, don't swim in it. You may be able to have the water tested. Call your local health department for details.

First of all where -: -the pond or lake. If on public land you should check with the county public health dept. the test waters for safety, mainly if frequented by the public. If private you can bring a sample to them & they will test it 4 U. They may or may not charge U. In larger counties maybe free, just as they May your well or tap water.